This is my guess too. Some bags are taped or have the round clear stickers sealing the flap, but some items have the heat sealed seam along the edges and you have to rip it open. I’ve received bras sealed this way against a piece of cardboard.They are probably rewrapping them. You think it’s a new/unused item because it’s a fully sealed plain plastic bag. If it was direct from the manufacture the plastic bag would most likely have the brands logo and barcode on it. Not all brands put their logo on the plastic bag but the higher end items generally will because it’s a part of buying the higher end experience. If it was rewrapped it’s a soft plastic (similar to plastic wrap you use in the kitchen) and can be sealed with heat. If you google wrapping equipment or shrink wrapping equipment you’ll get a better idea of what I’m talking about.
Employees should be inspecting items before sending them off but I imagine they’re busy. Last year around Nsale I received someone’s returned baby blanket with the original gift message still on the packaging